Vast quantities of oil and gas are held in deposits where they cannot be produced by conventional drilling. When the world price of oil is high, it becomes more profitable to find ways of exploiting these unconventional fossil fuel resources. For example:

Tar sands are deposits of heavy oil that are too thick, or viscous, to flow. These deposits form in the same manner as conventional deposits of oil, but microbes at shallow depths break down the light fraction of the oil. Energy developers extract oil from tar sands by injecting hot steam, which heats the sands and makes the tar less viscous so that it can be pumped out. Because generating the steam consumes large amounts of energy, tar sands are economically viable only when oil prices are high. The largest tar sand deposits in the world lie in the Athabasca region of Alberta, Canada, where production has increased rapidly in recent years (Fig. 10).

Oil shales are tight source rocks that are not permeable enough to pump the oils out directly. Potential technologies for extracting shale oil include fracturing and igniting the shales, causing the kerogen to mature and migrating the light oil fraction to pumping stations. To date, however, only a very limited amount of oil has been recovered from shales in pilot studies. The United States has huge reserves of shale oil, which could extend our national oil supply by decades to a century if technologies are developed to harvest them economically.

Natural gas can be extracted from coal, typically by pumping the gas directly from subsurface coal deposits (coal-bed methane). Alternatively, coal can be processed to produce syngas (coal gasification), which can also be used as a source of energy. Coal gasification offers tremendous promise, not only because it offers an opportunity to tap our domestic coal reserves but because pollutants and carbon dioxide can be removed from the gas before it is burned.

Gasification technology has already been commercialized for industrial purposes but has not been pursued for energy to date because it is more expensive than using coal directly as fuel. However, spurred by rising oil prices and concerns about emissions from coal combustion, several U.S. companies have announced plans to build coal gasification power plants. If these plants include the capacity to capture and store carbon dioxide emissions (which would increase their capital costs still further), they could help to make coal a more environmentally acceptable energy source.

Huge reserves of gas also occur trapped in ice within shallow sediments, both in permafrost and deep undersea environments. No affordable technology has been developed yet to harvest these broadly distributed methane hydrates or clathrates, but they may prove to be an important potential energy source because of their abundance and because natural gas burns more cleanly than other fossil fuels. These deposits are also under study because methane is a powerful greenhouse gas, so if they were to be vented to the atmosphere (for example, if frozen tundra thaws as Earth's surface temperature rises), they could substantially increase the rate of global climate change. Finding ways to develop methane hydrates and prevent uncontrolled releases thus would have both energy and climate change benefits.